Equilibrium thinking in Haber–Bosch:\nWhich set of operating conditions favors the maximum equilibrium yield of ammonia (N2 + 3 H2 ⇌ 2 NH3)?

Difficulty: Easy

Correct Answer: High pressure, high reactants concentration, low temperature

Explanation:


Introduction / Context:
The Haber–Bosch synthesis of ammonia is governed by equilibrium and kinetics. Because 4 moles of reactants form 2 moles of product, and the reaction is exothermic, Le Chatelier’s principle guides the selection of pressure, temperature, and reactant partial pressures to maximize ammonia equilibrium yield at the reactor outlet.


Given Data / Assumptions:

  • Stoichiometry: N2 + 3 H2 ⇌ 2 NH3; Δn(gas) = −2.
  • Reaction is exothermic (heat-releasing).
  • Question targets equilibrium yield, not rate alone.


Concept / Approach:
Since the reaction reduces gas moles, higher total pressure shifts equilibrium toward ammonia. Because the reaction is exothermic, lower temperature favors products, although kinetics slow at too-low temperatures; industrial practice balances both by using catalysts and moderate temperatures. High reactant partial pressures (i.e., high reactant concentration in the feed) also push equilibrium toward NH3. Therefore, the best combination is high pressure, high reactant concentration, and low temperature.


Step-by-Step Solution:
Apply Le Chatelier: increase pressure to favor the side with fewer moles (NH3).Lower temperature to favor exothermic product formation.Increase reactant partial pressures to drive the equilibrium toward ammonia.Combine these to select: high P, high reactants, low T.


Verification / Alternative check:
Plant practice uses elevated pressure (often 100–200 bar historically, lower in modern designs), catalyst beds, interbed cooling, and recycle to approach favorable equilibrium while retaining adequate reaction rates.


Why Other Options Are Wrong:
Low pressure works against equilibrium; high temperature lowers equilibrium NH3 even if rate improves; low reactant concentration reduces NH3 partial pressure driving force.


Common Pitfalls:
Confusing rate and equilibrium: very low temperature is favorable to equilibrium but too low harms kinetics—industrial conditions are a compromise, but for equilibrium yield alone, low temperature is preferred.


Final Answer:
High pressure, high reactants concentration, low temperature

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